Pigmentary glaucoma results from destruction of the pigment epithelia of the iris. A defect in the gene encoding a pigment-cell associated glycoprotein, Nmb was recently identified in mice prone to pigmentary glaucoma with iris pigment dispersion, similar to human Pigment Dispersion Syndrome. Nmb mRNA is expressed in a number of tissues, including pigment cells, but analyses of its subcellular localization and function have not been described. Several indirect lines of evidence suggest that Nmb plays a role in the structure and/or function of melanosomes, the organelles in which melanins are synthesized and stored. First, defects in the gene encoding a pigment cell-specific melanosomal protein, Tyrp1 , were identified in mice with another form of pigmentary glaucoma. Second, reduced pigmentation alleviated the symptoms of Nmb- and Tyrp1-deficient mice. Third, Nmb bears striking homology to a pigment cell-specific glycoprotein, Pme117, that is known to regulate pigmentation and bind to melanin intermediates. We have shown that Pmel17 is a morphogenetic component of melanosomes and their precursors in epidermal melanocytes, required for inducing the formation of fibrils upon which melanins are deposited; proteolytic processing of Pmel17 is essential for its function. We hypothesize that Nmb also localizes to melanosomes and functions in melanosome biogenesis, melanin production, or detoxification of melanin intermediates. We further hypothesize that alterations in Nmb expression or splice form usage will be prevalent among human glaucoma patients. We will address these hypotheses with the following specific aims: 1. Determine the subcellular localization of Nmb within melanocytes and retinal pigment epithelium and characterize its oligosaccharide and protein processing en route to its destination. 2. Determine whether Nmb interacts with Pmel17 and/or functions in melanosome morphogenesis. 3. Determine the range of Nmb localization and splice variation in eye tissue of disease-free donors and pigmentary glaucoma patients.